Cellular wireless systems are used to communicate voice and data with cellular telephones, personal digital assistants (PDAs), cellular modems, and other mobile devices. Such cellular wireless systems typically include a radio access network (RAN) that wirelessly communicates with such mobile devices. The wireless network can be connected to the Internet and also to the Public Switched Telephone Network (PSTN), allowing for users located within the coverage area(s) to communicate both with the PSTN and the Internet using such mobile devices. Thus, a user can retrieve information from the Internet (or send information to the Internet) in a data session where the mobile device exchanges packets of data with Internet servers through the wireless network. A user can also receive and/or place calls over the PSTN using such a mobile device.
Cellular wireless systems typically include a number of transceiver stations for wirelessly communicating data and/or voice information with mobile devices over radio channels. The transceiver stations each include a combination of directional and/or non-directional antennas that radiate to define a radiation pattern. The radiation patterns of the transceiver stations thus define geographic coverage areas in which mobile devices are able to send and receive signals with sufficient signal strength to communicate with the RAN. The coverage areas divide the cellular wireless system geographically into a number of cells and mobile devices within each coverage area communicate with the RAN via a respective transceiver station for that coverage area. Accordingly, transceiver stations are situated to provide coverage in geographic regions where coverage is desired.
The transceiver stations for each coverage area emit a pilot signal or reference signal. The pilot signals or reference signals are received by mobile devices to inform the mobile devices of the coverage areas available for communicating with the RAN. The mobile devices use the pilot signals or reference signals to identify coverage areas to join and then seek service on the RAN on one or more coverage areas. Coverage areas of nearby, or neighboring, transceiver stations can overlap geographically such that mobile devices may generally detect signals (e.g., pilot or reference signals) from more than one coverage area while in such overlapping regions. Signals from different coverage areas are distinguishable from one another to allow mobile devices in such overlapping regions to distinguish signals from one coverage area from those of another. For example, signals from different coverage can have different encoding parameters, such as in a code division multiple access (CDMA) architecture, or different frequencies, such as in an orthogonal frequency-division multiplexing (OFDM) architecture used in a long term evolution (LTE) network.
To provide substantially continuous network service to a mobile device passing between coverage areas, the RAN is configured to transfer (hand off) ongoing communications with such a mobile device between the coverage areas. For example, a mobile device can pass from a first coverage area to a second coverage area while remaining connected to the RAN. In such an example, the RAN transfers ongoing communications with the mobile device from a transceiver station for the first coverage area to a transceiver station for the second coverage area. Typically such transfers (or handoffs) occur while the mobile device is in an overlapping region where signals from the first and second coverage areas are both detected. In other words, such an overlapping region is an area where the mobile device is able to send and receive signals with sufficient signal strength to communicate with both the transferred-from transceiver station (for the first coverage area) and the transferred-to transceiver station (for the second coverage area).
Procedures for initiating and managing transceiver station handoff vary depending on the network architecture employed. Generally, each mobile device in the RAN regularly monitors signal strengths from pilot or reference signals emitted from its presently active coverage area as well as from neighboring coverage areas. The mobile device sends a message that includes an indication of the measured signal strengths. The message is received by the transceiver station, and the RAN determines whether to instruct the mobile device and transceiver stations to hand off an ongoing call to one of the neighboring coverage areas. Such hand off determinations can be made by a centralized radio network controller (RNC), such as in a CDMA system, or by distributed network controllers co-located with the transceiver stations, such in an LTE system including evolved Node B (eNB) transceiver stations. In some instances, the mobile device may only send such a signal strength report upon detecting a signal strength from a neighbor that satisfies a trigger, such as a signal strength that exceeds a predetermined threshold level, or a signal strength that exceed those of a currently active coverage area.
Each mobile device can store a list of neighboring coverage areas (a neighbor list) to scan for handoff. The neighbor list can include a listing of coverage areas surrounding the currently active coverage area and an indication of how to identify pilot signals or reference signals from such neighboring coverage areas (such as an encoding parameter or frequency channel). The RAN may provide such a neighbor list to each mobile device upon initial connection with the RAN and establishment of an active coverage area. The RAN may also provide an updated list to each mobile device upon handoff to another coverage area.
Due to variations in development of cellular wireless service amongst different providers and/or different wireless standards, transceiver stations in different regions may operate at different carrier frequencies (different channels). For instance, commonly employed radio frequency communication channels include channels in the 800 megahertz (MHz) band, 850 MHz band, 1800 MHz band, and 1900 MHz frequency band, among others. As a result, some mobile devices include appropriate interfaces to tune to one or more channels. For example, a “quad-band” mobile phone includes a receiver to communicate on channels in four different frequency bands, such as channels in the 800, 850, 1800, and/or 1900 MHz bands. As a multi-band mobile device moves from a coverage area operating in one band to a coverage area operating in another band, the mobile device wireless interface can transition from one channel to the other in order to facilitate handoff.
In operation then, a mobile device can detect a pilot or reference signal to identify a coverage area to join. The mobile device joins a coverage area, which may be referred to herein as an active coverage area. The mobile device is provided with a neighbor list identifying neighboring coverage areas to scan for handoff, and the neighboring coverage areas may operate on more than one carrier frequency. A call such as a voice call or data session can be initiated, and signals for communicating voice or data information are communicated between the mobile device and a transceiver station for the active coverage area. While the call is ongoing, the mobile device scans pilot signals or reference signals from the neighboring coverage areas and measures signals strengths. To initiate handoff, the mobile device sends a message indicating the measured signal strengths. A centralized or distributed controller system for the RAN evaluates the measured signal strengths in view of network loading and determines whether to instruct the wireless system to hand off communication to a neighboring coverage area. The RAN then sends instructions to the mobile device and to any affected transceiver stations to instruct that the communication of the active call be handed off.